In the past few years endeavors to uncover the secrets of control and function of the eucaryotic genome have heightened. In this time many new and powerful techniques have been developed: DNA sequencing, cloning etc. However, while these new techniques have done much for the advancement of the fine structure of specific eucaryotic genes, none allows the complete study of a system as to the control of transcription and translation of a specific portion of the eucaryotic genome. Puffing in the chromosomes of Chironomus enables direct visualization of gene expression during differentiation. The large puffs, the Balbiani Rings, of Chironous comprise a system in which a portion of the intact genome has been amplified. The transcription product, 75 S RNA, of this specific area has been well studied. However the function of the giant 75 S RNA is still undetermined, though a messenger role is strongly implied. To complete the study of the function of the Balbiani Ring II locus it is necessary to positively identify the gene product of this BR region. Correlative data relate the salivary polypeptides with this 75 S RNA molecule. This study will determine whether the apparent relationship is causal. This is to be done with the use of in vivo translation of entire m-RNA preparation from salivary glands as well as of the 75 S RNA specifically. This system allows a unique opportunity for a complete analysis, structural and functional, of gene expression at a specific locus, during differentiation.